Methods for growing low-resistivity tungsten for high aspect ratio and small features

ABSTRACT

The present invention addresses this need by providing methods for depositing low resistivity tungsten films in small features and features having high aspect ratios. The methods involve depositing very thin tungsten nucleation layers by pulsed nucleation layer (PNL) processes and then using chemical vapor deposition (CVD) to deposit a tungsten layer to fill the feature. Depositing the tungsten nucleation layer involves exposing the substrate to alternating pulses of a boron-containing reducing agent and a tungsten-containing precursor without using any hydrogen gas, e.g., as a carrier or background gas. Using this process, a conformal tungsten nucleation layer can be deposited to a thickness as small as about 10 Angstroms. The feature may then be wholly or partially filled with tungsten by a hydrogen reduction chemical vapor deposition process. Resistivities of about 14 μΩ-cm for a 500 Angstrom film may be obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 12/030,645 (now U.S. Pat. No. 7,955,972), filedFeb. 13, 2008, titled “Methods for Growing Low-Resistivity Tungsten forHigh Aspect Ratio and Small Features,” which claims the benefit of USProvisional Patent Application No. 60/904,015, filed Feb. 27, 2007, andis also a continuation-in-part of U.S. patent application Ser. No.11/265,531 (now U.S. Pat. No. 7,589,017), filed Nov. 1, 2005, titled“Methods For Growing Low-Resistivity Tungsten Film,” which in turn is acontinuation-in-part of U.S. patent application Ser. No. 10/815,560 (nowU.S. Pat. No. 7,262,125), filed Mar. 31, 2004, titled “Method of FormingLow-Resistivity Tungsten Interconnects,” which in turn is acontinuation-in-part of U.S. patent application Ser. No. 10/649,351 (nowU.S. Pat. No. 7,141,494), filed on Aug. 26, 2003, titled “Method forReducing Tungsten Film Roughness and Improved Step Coverage,” which isin turn a continuation-in-part of U.S. patent application Ser. No.09/975,074 (now U.S. Pat. No. 6,635,965), filed Oct. 9, 2001, which inturn claims the benefit of U.S. Provisional Patent Application No.60/292,917, filed May 22, 2001. All of these applications areincorporated herein by reference in their entireties.

FIELD OF INVENTION

This invention relates to methods for preparing tungsten films. Theinvention is particularly useful for integrated circuit applicationsthat require thin tungsten films having low electrical resistance.

BACKGROUND

The deposition of tungsten films using chemical vapor deposition (CVD)techniques is an integral part of many semiconductor fabricationprocesses. The tungsten films may be used to produce low resistivityelectrical connections in the form of horizontal interconnects, viasbetween adjacent metal layers, and contacts between a first metal layerand the devices on the silicon substrate. In a conventional tungstendeposition process, the wafer is heated to the process temperature in avacuum chamber, and then a very thin portion of tungsten film, whichserves as a seed or nucleation layer, is deposited. Thereafter, theremainder of the tungsten film (the bulk layer) is deposited on thenucleation layer. Conventionally, the bulk layer is formed by thereduction of tungsten hexafluoride (WF₆) on the growing tungsten layer.The bulk layer is generally deposited more rapidly than the nucleationlayer, but cannot be produced easily and reliably without first formingthe nucleation layer.

CVD and other deposition methods can be used to form a thin tungstennucleation layer. In a CVD technique, the WF₆ and reducing gas aresimultaneously introduced into the reaction chamber. This produces acontinuous chemical reaction of mixed reactant gases that continuouslyforms tungsten film on the substrate surface. In a typical example, CVDnucleation layers are deposited from WF₆-SiH4 with an argon carrier gas.In some instances, CVD nucleation performance is enhanced by thepresence of H₂ in carrier gas mixture. Note that the WF₆-SiH₄ reactionis much faster than the WF-H₂ reaction due to lower activation energyand greater reactivity. Other deposition methods such as atomic layerdeposition (ALD) and pulsed nucleation layer (PNL) techniques may alsobe used to form nucleation layers.

Advancing technology requires that tungsten electrical connects beincreasingly thin yet maintain very low resistance. Hence, it iscritical that tungsten deposition process provide tungsten having verylow resistivity. CVD-deposited bulk tungsten makes up most of the film,but how the CVD film grows depends on the nucleation film. Although CVDand other methods have been able to deposit nucleation layers, theirability to provide nucleation layers for the deposition of lowresistivity tungsten in smaller features with high aspect ratios islimited. What are therefore needed are improved methods for forming aconformal nucleation layer that will lead to low resistivity tungstenfilms with good step coverage for small features.

SUMMARY OF INVENTION

The present invention addresses this need by providing methods fordepositing low resistivity tungsten films in small features and featureshaving high aspect ratios. The methods involve depositing very thintungsten nucleation layers by pulsed nucleation layer (PNL) processesand then using chemical vapor deposition (CVD) to deposit a tungstenlayer to fill the feature. Depositing the tungsten nucleation layerinvolves exposing the substrate to alternating pulses of aboron-containing reducing agent and a tungsten-containing precursorwithout using any hydrogen gas, e.g., as a carrier or background gas.Using this process, a conformal tungsten nucleation layer can bedeposited to a thickness as small as about 10 Angstroms. The feature maythen be wholly or partially filled with tungsten by a hydrogen reductionchemical vapor deposition process. Resistivities of about 14 μΩ-cm for a500 Angstrom film may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description can be more fully understood whenconsidered in conjunction with the drawings in which:

FIG. 1 shows simple cross-sectional diagrams of tungsten deposition inlow and high aspect ratio features.

FIGS. 2A-2C are simple cross-sectional diagrams illustrating depositionof tungsten in a narrow width feature using various processes.

FIG. 3 is a process flow sheet showing relevant operations of methodsaccording to various embodiments of the present invention.

FIG. 4 is a simple cross-sectional diagram of a film stack including atitanium adhesion layer together with a tungsten nucleation layer and atungsten bulk layer formed in accordance with this invention.

DETAILED DESCRIPTION

Introduction

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention,which pertains to forming thin tungsten films. Preferred methods involvepulsed nucleation layer (PNL) deposition techniques, which will bedescribed in detail below. Modifications, adaptations or variations ofspecific methods and or structures shown and discussed herein will beapparent to those skilled in the art and are within the scope of thisinvention.

In a PNL technique, pulses of the reducing agent, purge gases, andtungsten-containing precursors are sequentially injected into and purgedfrom the reaction chamber. The process is repeated in a cyclical fashionuntil the desired thickness is achieved. PNL is similar to atomic layerdeposition techniques reported in the literature. PNL is generallydistinguished from atomic layer deposition (ALD) by its higher operatingpressure range (greater than 1 Torr) and its higher growth rate percycle (greater than 1 monolayer film growth per cycle). In the contextof this invention, PNL broadly embodies any cyclical process ofsequentially adding reactants for reaction on a semiconductor substrate.Thus, the concept embodies techniques conventionally referred to as ALD.

The present invention involves forming a tungsten film by way of atungsten nucleation layer. In general, a nucleation layer is a thinconformal layer which serves to facilitate the subsequent formation of abulk material thereon. The nucleation layer may be formed using one ormore PNL cycles. Additional discussion regarding PNL type processes canbe found in the related U.S. patent application Ser. Nos. 10/435,010,10/649,351 and 10/690,492, 11/265,531, incorporated herein by reference.

As features become smaller, the tungsten (W) contact or line resistanceincreases due to scattering effects in the thinner W film. Whileefficient tungsten deposition processes require tungsten nucleationlayers, these layers typically have higher electrical resistivities thanthe bulk tungsten layers. Thus, to keep the electrical resistance of theoverall tungsten film (tungsten nucleation layer and bulk tungsten) low,the tungsten nucleation layer should be kept as thin as possible. Asimplified equation describing the total resistance of a tungsten layeris:R _(total) =R _(bulk) +R _(nucleation)=ρ_(bulk)(L _(bulk)/A)+ρ_(nucleation)(L _(nucleation) /A)

This is shown in the above simplified equation of total resistance,R_(total), where ρ is the resistivity of the material, L is the lengthof the layer in the direction of the current flow and A is thecross-sectional area perpendicular to current flow. (It should be notedthat contributions to the total resistance are neglected in the aboveequation). Because the ρ_(nucleation)>ρ_(bulk), the thickness of thenucleation layer should be minimized to keep the total resistance as lowas possible. On the other hand, the tungsten nucleation should besufficiently thick to fully cover the underlying substrate to supporthigh quality bulk deposition. So, to achieve an optimal thickness, thetungsten nucleation layer may be formed in one or more PNL depositioncycles.

For narrow width and/or high aspect ratio and thin features, obtainingthin nucleation layers is even more critical. FIG. 1 shows a relativelylow aspect ratio feature 101 is shown in comparison to a relatively highaspect ratio feature 103. (These features are not drawn to scale but toillustrate the qualitative difference between nucleation layers in highand low aspect ratio features). Here, the thickness t is the same forboth features, but the width W2 of feature 103 is much less than widthW1 of feature 101 and the nucleation layer takes up a significantlyhigher percentage of the total volume of the feature. As a result, thenucleation layer has a much high relative contribution to the overallresistance of the feature. Thus, it becomes important to reduce thenucleation layer thickness (for example from a 50 A film to <30 A) forsmall features (for example a feature having a 10:1 aspect ratio or 400A opening) in order to reduce the overall stack resistivity. However,current PNL nucleation layers for standard and low resistivity Wprocesses has a limitation of nucleation layer thickness of around 30 Ain order to obtain good step coverage and plug fill. Reducing thenucleation layer further using current processes results in degradationof the step coverage.

Thin and uniform nucleation layers are also important for obtaining highquality, void-free tungsten deposition in narrow width and/or highaspect ratio features. Depositing thin nucleation layers with previousprocesses resulted in nucleation layers only forming on the top of thefeature, without depositing a continuous nucleation layer in the bottomof the feature. Most tungsten fill processes fill in from the side aswell at the bottom of the feature, so that when there is no nucleationlayer at the bottom, CVD film cannot grow from the bottom or from thesidewall near the bottom of the feature. But the top will have somenucleation film, and CVD tungsten grows much faster on the top than thebottom sidewall due to unequal nucleation layer deposition. This causesa pinch off at the top leaving a void at the bottom. Previous processescompensated for this problem by doing a thicker nucleation layer. Butthe nucleation layer has higher resistivity and occupies a largerpercentage of the total volume when the feature is smaller as describedabove. Previous low resistivity tungsten nucleation layer processes(carried out at low temperature) were unable to deposit even thickernucleation layers at the bottom of high aspect ration and smallfeatures.

FIG. 2 shows tungsten deposition in high aspect ratio features usingvarious processes. First, in FIG. 2A, tungsten deposition is shown usinga low-resistivity tungsten process, which is able obtain good featurefill for aspect ratios of 10:1 or lower and features widths of aboveabout 400 Angstroms. This process involves one or more PNL cycles at lowtemperature (e.g., about 300 C) to deposit the nucleation layer followedby CVD bulk deposition. Multiple reducing agent pulse/tungsten-precursorpulse cycles are performed to deposit the nucleation layer. Hydrogen istypically run in the background and/or as a carrier gas (for thereducing agent and/or tungsten precursor) during the PNL process.Descriptions of various low resistivity tungsten deposition processesare in above-referenced U.S. application Ser. No. 11/265,531. Theseprocesses are able to good step coverage and deposit nucleation layershaving thicknesses as low between about 20 and 30 Angstroms. In higheraspect ratios and small features, however, even nucleation layers asthin as 20 Angstroms be too thick to achieve good gap fill. FIG. 2A is arough schematic of a feature with a nucleation layer of 30 Angstroms,which contains a void. (The schematic is not shown to scale).

Depositing thinner nucleation layers with these processes, e.g., byreducing the number of PNL steps, results in nucleation layers withoutgood step coverage. Tungsten distribution is uneven in the feature andmay not fully cover the feature. As a result, the nucleation layer isnot thick enough to support quality bulk deposition. FIG. 2B shows arough schematic of a feature with patchy nucleation layer deposition.Step coverage is very poor. Subsequent bulk deposition (now shown) isvery poor with voids.

The processes described herein provide very thin nucleation layershaving good step coverage—optimal characteristics. An example of aresulting feature is shown in FIG. 2C. The films also have lowerresistivities than previously obtainable. Although applicable to highaspect ratio features, the methods described herein may also be used inother applications to deposit ultra-thin tungsten nucleation layershaving low resistivity.

Embodiments described herein achieve good step coverage and lowresistivity W film with as little as 12 A of a nucleation layer for veryaggressive features. This is compared to conventional PNL nucleationdeposition processes that fail to give good step coverage and W plugfillwith such thin nucleation layers. In addition, the bulk resistivity ofthe resulting PNL nucleation film is less than that of the standard PNLprocess. The combination of lower bulk resistivity and thinnernucleation thickness gives lower stack resistivity than the standardprocess. For example, for a 20 Angstrom nucleation layer, the bulkresistivity using embodiments described herein is about 50 μΩ-cm,compared to about 75 μΩ-cm using the standard process.

The inventors have recognized that running hydrogen in the background oras a carrier gas—as is conventionally done in processes such asdescribed above with reference to FIGS. 2A—results in a higherdeposition rate and thicker nucleation layer deposited. This is trueeven at the low temperatures used to deposit low resistivity tungsten.It is believed that in essence the reaction becomes more CVD-like,resulting in decreased step coverage. Without being bound by aparticular theory it is believed that by alternating PNLboron-containing reducing agent/tungsten-containing precursor pulseswithout any hydrogen present, the reactants are pushed down better intohigh aspect ratio features.

FIG. 3 is a flow chart depicting one process flow in accordance withcertain embodiments of the present invention. Initially, a substrate isprovided and positioned in a reaction chamber as indicated by a processblock 301. As mentioned previously, in many embodiments the substrate isa partially fabricated electronic device (e.g., a partially fabricatedintegrated circuit). Specific applications of the invention are bedescribed further below. The substrate contains a feature that has ahigh aspect ratio and/or narrow width. High aspect ratios range from5:1-30:1. In certain embodiments, the aspect ratio is at least 10:1 or20:1. Features having widths as narrow as 300-400 Angstroms also benefitfrom this process. In some cases, both the feature has both a highaspect ratio and a narrow width, but features having only one of thesegeometric characteristics benefit from the processes. For example, inone embodiment, a low resistivity tungsten layer is deposited in afeature having a width around 500 Angstrom and aspect ratio of about30:1—a geometry at which high quality low resistivity tungsten byprocesses described above with reference to FIG. 2A is not possible.

Next, as indicated by a process block 303, the substrate is exposed to aboron-containing reducing agent to form a boron-containing layer on thesubstrate surface. For low-resistivity applications, boron-containingreducing agents are used. However, in certain embodiments, a silane maybe used in place of the boron-containing reducing agent to achieve goodstep coverage in high aspect ratio and/or small features, without thebenefit of the low resistivity tungsten.

The boron-containing layer is often a layer of elemental boron, thoughin some embodiments, it may contain other chemical species or impuritiesfrom the boron-containing species itself or from residual gases in thereaction chamber. Any suitable boron-containing species may be used,including borane (BH₃), diborane (B₂H₆), triborane, etc. Examples ofother boron-containing species include boron halides (e.g., BF₃, BCl₃)with hydrogen.

Substrate temperature is low—below about 350 C, for example betweenabout 250 and 350 C or 250 and 325 C. In certain embodiments, thetemperature is around 300 C. Frequently, diborane is provided from adiluted source (e.g., 5% diborane and 95% nitrogen). Diborane may bedelivered the reaction chamber using other or additional carrier gasessuch as nitrogen, argon, and/or silane. Importantly, no hydrogen isused.

Once the boron-containing layer is deposited to a sufficient thickness,the flow of boron-containing species to the reaction chamber is stoppedand the reaction chamber is purged with a carrier gas such as argon,hydrogen, nitrogen or helium. In certain embodiments, only argon is usedat the carrier gas. The gas purge clears the regions near the substratesurface of residual gas reactants that could react with fresh gasreactants for the next reaction step.

Referring back to FIG. 3, the next operation 305 involves contacting thesubstrate with a tungsten-containing precursor to form a portion of thetungsten nucleation layer. Any suitable tungsten-containing precursormay be used. In preferred embodiments the tungsten-containing precursoris one of WF₆, WCl₆ and W(CO)₆. The tungsten-containing precursor istypically provided in a dilution gas, such as argon, nitrogen, or acombination thereof. As with the boron-containing precursor pulse, thetungsten-containing precursor is delivered in a non-hydrogenenvironment. The substrate temperature is low—below about 350 C, forexample between about 250 and 350 C or 250 and 325 C. In certainembodiments, the temperature is around 300 C. Tungsten-containingprecursor dosage and substrate exposure time will vary depending upon anumber factors. In general, the substrate is preferably exposed untilthe adsorbed boron species is sufficiently consumed by reaction with thetungsten-containing precursor to produce a portion of the tungstennucleation layer. Thereafter, the flow of tungsten-containing precursorto the reaction chamber is stopped and the reaction chamber is purged.The resulting portion of tungsten nucleation layer deposited in oneboron-containing reducing agent/tungsten-containing precursor PNL cycleis typically about five Angstroms

Returning again to FIG. 3, the low temperature boron-containing reducingagent pulse and tungsten precursor pulse operations are repeated tobuild up the tungsten nucleation layer in an operation 307. Typicallybetween about 3-7 PNL cycles are required to deposit the very thinnucleation layer. Depending on the substrate, the first one or twocycles may not result in thickness gain due to nucleation deloay. Asdescribed previously, the tungsten nucleation layer should besufficiently thin so as to not unduly increase the overall tungstenfilm, but sufficiently thick so as to support a high quality bulktungsten deposition. The process described above is able to deposit atungsten nucleation layer that can support high quality bulk depositionas low as about 10 Angstroms in the high aspect ratio and/or narrowwidth feature. The thickness of the deposited nucleation layer istypically between about 10 and 30 Angstroms.

Temperature is one of the process conditions that affects the amount oftungsten deposited. Others include pressure, flow rate and exposuretime. As discussed above, in many embodiments, maintaining temperaturesat or below about 350 C results in less material deposited during acycle than in previously described tungsten nucleation layer depositionprocesses. This in turn provides lower resistivity. These PNL steps aregenerally performed at low temperature, e.g., below about 450 C and insome embodiments below about 350 C, and may result in less materialdeposited during a cycle than in previously described tungstennucleation layer deposition processes. In some embodiments, temperaturesmay be about 200 C. Because of this, for most applications of theinvention, the desired thickness of the tungsten nucleation layerrequires more than one PNL cycle.

Once the tungsten nucleation layer has reached the desired thickness,the tungsten nucleation layer is fully formed and next a bulk tungstenlayer is deposited in the feature in a process operation 309. In manyembodiments the bulk tungsten is deposited using a CVD process. CVDprocesses rapidly produce low resistivity films. Any suitable CVDprocess may be used with any suitable tungsten-containing precursor. Insome embodiments the same tungsten-containing precursor used in the PNLprocesses for forming the tungsten nucleation layer is use—typically oneof WF₆, WCl₆ and W(CO)₆. Frequently, the CVD process is performed usinga mixture of molecular hydrogen and one or more of these precursors. Inother embodiments, the CVD process may employ a tungsten precursortogether with silane or a mixture of hydrogen and silane or a mixture ofhydrogen and borane (such as diborane). Non-CVD process can also beemployed to form the bulk layer. These include ALD/PNL and physicalvapor deposition (PVD).

The bulk tungsten can be deposited to any thickness. Tungsteninterconnect lines for integrated circuit applications may have a totalthickness (tungsten nucleation layer and bulk tungsten) of between about20 and 1,000 Angstroms. For a typical bit line, the total tungsten filmthickness typically is no more than about 600 Angstroms. The resultingtungsten film will preferably have a resistivity of no greater thanabout 30 μΩ-cm. Resisitivity depends on how much of the total thicknessis due to the nucleation layer. The resistivity for 500 A film(nucleation+CVD tungsten) deposited using a PNL nucleation depositionprocess as described with reference to FIG. 2A is about 18 μΩ-cm. Usingthe process described above with reference to FIG. 3, the resistivityfor a 500 A film is about 14 μΩ-cm.

After the tungsten film is deposited to a sufficient thickness, theprocess flow of FIG. 1 is complete. While the process parameters can bevaried in order to influence the thickness of the nucleation layerformed at a particular cycle of the process, it has been found thatdepositing less material per cycle may improve step coverage, adhesionand/or resistivity. For this reason, deposition temperature ismaintained at or less than about 350 C, though as discussed above,higher temperatures may also be used during the PNL deposition stepsaccording to some embodiments.

Using the methods described herein, ultra-thin W nucleation layer (e.g.,<20 A) that still give good plugfill are achieved. This was previouslyunattainable except with a nucleation layer >20 A. In addition, due tolower bulk resistivity of the PNL film itself, the overall contactresistance is improved further. Overall contact and trench lineresistance may be reduced by 50%.

The following processes are examples of processes that may be used tofill high aspect ratio features with tungsten, resulting in good gapfilland low resisitivity.

EXAMPLE 1

A W nucleation layer was formed in a feature having an AR of 30:1 and a0.15 μm opening the following pulsed nucleation layer process:

-   -   5 Torr, 300 C, B₂H₆ dose (7.5 sec.), 10 sec. purge, WF₆ dose (10        sec)    -   A W bulk layer was then formed using a CVD process (H₂-WF₆) at        80 Ton and 395 C.

The thickness of the nucleation layer was about 80 Angstroms. Totalthickness was 2000 Angstroms. Resisitivity was of the film as measuredon a blanket wafer was 11 μΩ-cm. Step coverage was greater than 90%.

EXAMPLE 2

A W nucleation layer was formed in a feature having a 10:1 AR and a0.075 μm opening using the following pulsed nucleation layer process:

-   -   40 Ton, 300 C, B₂H₆ dose (1 sec.), 3 sec. purge, WF₆ dose (0.5        sec), 3 sec purge (5 cycles)    -   W bulk layer was then formed using a CVD process (H₂-WF₆) at 40        Ton and 395 C.

The thickness of the nucleation layer was about 20 Angstroms. Totalthickness was 2000 Angstroms. Resisitivity was of the film as measuredon a blanket wafer was 11 μΩ-cm. Step coverage was greater than 90%.

Apparatus

The methods of the invention may be carried out in various types ofdeposition apparatus available from various vendors. Examples ofsuitable apparatus include a Novellus Concept-1 Altus, a Concept 2Altus, a Concept-2 ALTUS-S, a Concept 3 Altus deposition system, or anyof a variety of other commercially available CVD tools. In some cases,the process can be performed on multiple deposition stationssequentially. See, e.g., U.S. Pat. No. 6,143,082, which is incorporatedherein by reference for all purposes. In some embodiments, the pulsednucleation process is performed at a first station that is one of two,five or even more deposition stations positioned within a singledeposition chamber. Thus, the reducing gases and the tungsten-containinggases are alternately introduced to the surface of the semiconductorsubstrate, at the first station, using an individual gas supply systemthat creates a localized atmosphere at the substrate surface.

In one example, after a first thickness of tungsten deposited, the waferis moved to a second deposition station and a new wafer is moved intoplace on the first station. The wafers may be indexed from onedeposition station to the next to enable parallel wafer processing afterone or more repetitions of the cycle. The full thickness of the tungstenfilm is achieved by additional cycles with alternating reducing gasesand tungsten-containing gases at one or more of the other depositionstations. This is repeated until all substrates are coated to thedesired thickness. It is the sum of these individual depositions thatforms the total amount of tungsten nucleation layer deposited. Anynumber of deposition stations, each capable of having a localizedatmosphere isolated from adjacent stations, is possible within thesingle chamber.

The invention also provides for a deposition chamber in whichalternating deposition stations are dedicated to deliver eithertungsten-containing gases or reducing gases. More specifically, thedeposition stations in the chamber are separated into two groups withthe first group dedicated to delivery of the reducing gases and thesecond group for introducing tungsten-containing gas. These stationsalso can provide for the simultaneous delivery of carrier gases and/orhydrogen with the dedicated gases. Thus, tungsten is deposited by movingwafers from station to station such that the wafer is sequentiallyexposed to the reducing gases and then the tungsten-containing gasesuntil the desired thickness of tungsten is obtained.

Applications

The present invention may be used to deposit thin, low resistivitytungsten layers for many different applications. One preferredapplication is for interconnects in integrated circuits such as memorychips and microprocessors. Interconnects are current lines found on asingle metallization layer and are generally long thin flat structures.These may be formed by a blanket deposition of a tungsten layer (by aprocess as described above), followed by a patterning operation thatdefines the location of current carrying tungsten lines and removal ofthe tungsten from regions outside the tungsten lines.

A primary example of an interconnect application is a bit line in amemory chip. Of course, the invention is not limited to interconnectapplications and extends to vias, contacts and other tungsten structurescommonly found in electronic devices. In general, the invention findsapplication in any environment where thin, low-resistivity tungstenlayers are required.

The invention is also focuses on low resistivity, tungsten layers havingrelatively thin proportions, typically on the order of 500 angstroms orless, preferably, 300 angstroms or less. But more generally, theinvention applies to a broader range of tungsten layers, including thosewith thicknesses of between about 5 angstroms to 1000 angstroms.

Another parameter of interest for many applications is a relatively lowroughness of the ultimately deposited tungsten layer. Preferably, theroughness of the tungsten layer is not greater than about 10% of thetotal thickness of the deposited tungsten layer, and more preferably notgreater than about 5% of the total thickness of the deposited tungstenlayer. The roughness of a tungsten layer can be measured by varioustechniques such as atomic force microscopy.

FIG. 4 is a cross-section illustration of a film stack that can beformed using methods of the invention. The film stack may representinterconnect applications as described previously. The film stack ofFIG. 4 is formed in an underlying substrate having a feature tungsten isto be deposited. The feature which may be a single component or morecommonly a complex multi-feature structure having various conductive,insulating, and semiconductor components. For example, the substrate mayhave a top layer comprising silicon or a dielectric such as silicondioxide. Contacting the substrate is, in the following order, a titaniumlayer 403, a titanium nitride layer 405, a tungsten nucleation layer 407(formed in accordance with this invention) and a tungsten bulk layer409. Titanium layer 403 is typically deposited by a CVD process whichprovides reasonably good adhesion to the underlying substrate 401.Titanium nitride layer 405 is typically deposited using CVD or PVDmethods and is used to protect the underlying titanium and/or siliconfrom exposure to tungsten hexafluoride (WF₆) during subsequent tungstendeposition. It has been found that WF₆ reacts very aggressively andsometimes explosively with titanium. Tungsten nucleation layer 407 andtungsten bulk layer 409 are formed in accordance with the methods of thepresent invention as described above. In interconnect applications asdescribed above, layers 403, 405, 407 and 409 are all etched to forminterconnect lines. In another embodiment, a tungsten nitride layer isemployed instead of the Ti/TiN layer.

Other Embodiments

While this invention has been described in terms of several embodiments,there are alterations, modifications, permutations, and substituteequivalents, which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and apparatuses of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, modifications, permutations, and substituteequivalents as fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. A method of forming a tungsten layer in featureon a substrate, said feature including a feature opening, the methodcomprising: forming a conformal tungsten nucleation layer in at leastthe feature by exposing the substrate to alternating pulses of aboron-containing reducing agent and a tungsten containing precursor at asubstrate temperature of between about 250° C.-350° C., wherein nohydrogen is flowed during or between the pulses.
 2. The method of claim1 wherein the feature opening prior to forming the conformal nucleationlayer is less than about 400 Angstroms.
 3. The method of claim 2 furthercomprising, after forming the conformal nucleation layer, simultaneouslyexposing the substrate to a tungsten-containing precursor and a secondreducing agent to thereby deposit a bulk tungsten layer by a chemicalvapor deposition process in at least the feature.
 4. The method of claim3 wherein the second reducing agent is hydrogen.
 5. The method of claim1 wherein the substrate temperature during formation of the tungstennucleation layer is between about 250° C. and 325° C.
 6. The method ofclaim 1 wherein the substrate temperature during formation of thetungsten nucleation layer is about 300° C.
 7. The method of claim 1wherein the tungsten nucleation layer is deposited to a total thicknessof between about 10-20 Angstroms.
 8. The method of claim 1 wherein thetungsten nucleation layer is deposited to a total thickness of betweenabout 10-15 Angstroms.
 9. The method of claim 1 wherein the aspect ratioof the feature is at least about 10:1.
 10. The method of claim 1 whereinthe feature has an opening width of no more than about 300 Angstroms.11. The method of claim 3 wherein the substrate temperature during thechemical vapor deposition process is about 395° C.
 12. A method offorming a tungsten layer comprising in a feature on a substrate: forminga conformal tungsten nucleation layer in at least the feature byalternating pulses of a boron-containing reducing agent and a tungstencontaining precursor at a substrate temperature of between about 250°C.-350° C., wherein no hydrogen is flowed during or between the pulses;and after forming the conformal nucleation layer, substantially fillingthe feature with a tungsten bulk layer by simultaneously exposing thesubstrate to a tungsten-containing precursor and hydrogen therebydeposit tungsten by a chemical vapor deposition process in at least thefeature.
 13. The method of claim 12 wherein the substrate temperatureduring formation of the tungsten nucleation layer is between about 250°C. and 325° C.
 14. The method of claim 12 wherein the substratetemperature during formation of the tungsten nucleation layer is about300° C.
 15. The method of claim 12 wherein the tungsten nucleation layeris deposited to a total thickness of between about 10-20 Angstroms. 16.The method of claim 12 wherein the tungsten nucleation layer isdeposited to a total thickness of between about 10-15 Angstroms.
 17. Themethod of claim 12 wherein the aspect ratio of the feature is at leastabout 20:1.
 18. The method of claim 12 wherein the feature has anopening width of no more than about 400 Angstroms.
 19. The method ofclaim 12 wherein the substrate temperature during the chemical vapordeposition process is about 395° C.
 20. The method of claim 12 whereinthe total thickness of tungsten deposited is about 500 Angstroms. 21.The method of claim 20 wherein the resistivity of the deposited film isno more than about 14 μΩ-cm.